1. Field of the Invention
The present invention relates to a gigabit Ethernet-passive optical network (hereinafter referred to as “GE-PON”), and more particularly to a data transmission method used in a GE-PON architecture.
2. Description of the Related Art
In order to construct a network from a telephone office to a building or home, diverse network structures such as an xDSL (x-digital subscriber line), HFC (hybrid fiber coax), FTTB (fiber to the building), FTTC (fiber to the curb), FTTH (fiber to the home), etc., with different multiplexing schemes have been implemented. Among such diverse network structures, FTTx (x=B, C, H) may be classified into an active FTTx, which is configured by an active optical network (hereinafter referred to as “AON”) construction, and a passive FTTx, which is configured by a passive optical network (hereinafter referred to as “PON”) construction. The later construction has been favored due to its economical and simple network construction, which includes a point-to-multipoint topology incorporating the passive elements.
The passive optical network (PON) is formed of a distributed tree structure topology in which one optical line termination (hereinafter referred to as “OLT”) is connected to a plurality of optical network units (hereinafter referred to as “ONU”) using a 1×N passive optical splitter. Recently, in the field of ITU-T (International Telecommunication Union-Telecommunication section), the contents of standardization of an asynchronous transfer mode-passive optical network (hereinafter referred to as “ATM-PON”) system have been proposed in ITU-T G.982, ITU-T G.983.1, and ITU-T G.983.3. Moreover, in the field of IEEE 802.3ah TF of the IEEE (Institute of Electrical and Electronics Engineers), a standardization work of a gigabit Ethernet based the PON (hereinafter referred to as “GE-PON”) is in progress.
The standardization of a point-to-point type gigabit Ethernet and MAC (medium access control) technology for ATM-PON has been completed, and the contents thereof are described in IEEE802.3z and ITU-T G.983.1. Also, U.S. Pat. No. 5,973,374 entitled, “Protocol for data communication over a point-to-multipoint passive optical network”, which was invented by Gigad Ghaib et al. and issued on Nov. 2, 1999, discloses in detail the MAC technology of ATM-PON.
FIG. 1 illustrates a conventional point-to-point type gigabit Ethernet standard frame structure. In the point-to-point type gigabit Ethernet whose standardization has been completed, but the functions required for a point-to-multipoint PON system have not yet been specified.
FIG. 2 illustrates a schematic construction of a conventional ATM-PON system. As shown, the ATM-PON system includes an optical line terminal (OLT) 10 for playing a central role of providing information to the respective subscribers of an accessed network. An optical distribution network (ODN) 16 is connected to the OLT 10 and has a tree topology structure. During operation, the ODN 16 serves to distribute downstream data frames transmitted from the OLT 10 and multiplexes upstream data frames to the OLT 10. A plurality of optical network units (ONUs) 12a, 12b and 12c receive and provide the downstream data frames to the terminal users 14a, 14b and 14c, and also transmit data outputted from the terminal users to the ODN 16 as the upstream data frames. Note that the terminal users 14a, 14b and 14c represent various types of network terminals that can be used in the PON including network terminals (NTs).
As shown in FIG. 2, the conventional ATM-PON system performs the upstream and downstream transmission of fixed data frame, in which ATM cells having a size of 53 bytes are grouped together. The OLT 10 is configured to insert the downstream data to be distributed to the respective ONUs 12i in the downstream cells. For the upstream transmission, OLT 10 accesses the data transmitted from the plurality of ONUs 12a, 12b and 12c in a time division multiplexing (TDM) manner. As the ODN 16, which is connected between the OLT 10 and the plurality of ONUs 12a, 12b and 12c, is a passive element, the OLT 10 is operative to prevent data from colliding in the passive element according to a virtual distance correction algorithm known as a ranging algorithm. In addition, in case of the downstream data transmission from the OLT 10 to the plurality of ONUs 12a, 12b and 12c, the OLT 10 receives from and sends to the ONUs 12a, 12b and 12c an encryption key for security and OAM (operations, administration and maintenance) messaging applications. In this regards, a corresponding data field is prepared in the dedicated ATM cell or a general ATM cell for receiving and sending messages at predetermined intervals in the upstream/downstream frames.
With the development of the Internet technology, a subscriber requires more and more bandwidths, thus demanding an end-to-end transmission in the gigabit Ethernet technology, which is relatively cheap and capable of securing a higher bandwidth (of about 1 Gbps) transmission when compared to the traditional ATM technology. The ATM technology requires relatively expensive equipments and has a bandwidth limitation (of 622 Mbps at maximum), and further must perform the segmentation of IP (internet protocol) packets. Accordingly, in the PON structure of the network, the Ethernet system is more favored over the ATM system.
The ATM-PON system, as described above, constructs upstream and downstream frames on the basis of ATM cells having a fixed size, and uses the time division multiplexing (TDM) scheme with respect to the upstream transmission in accordance with the tree structure of a point-to-multipoint connection.
In of the field of gigabit Ethernet, the standardization of the GE-PON structure is now in progress. Currently, the function and implementation of an Ethernet PON, which transmits the Ethernet frames through a PON, are in progress now in the field of GE-PON standard. For the upstream transmission in the tree-shaped PON structure as described above, data of the respective ONUs 12i are accessed in the TDM scheme, and a scheduling algorithm is required for transmitting signals to avoid collisions in the ODN 16, which is a passive element.
In case of downstream transmission of 155.52 Mbps in the ATM-PON, 56 ATM cells constitute one frame, and two out of these cells are used as downstream PLOAM cells for controlling a physical layer. The PLOAM cells transfers information to the OLT using a pre-defined message to control the OLT. If the upstream message in the form of a mini-slot transfers information of an ONU 12i is which is the same as that of cells where queues are accumulated to the respective registered ONU 12i, the OLT 10 transfers the bandwidth-allocated transmission grant signals to the ONUs for the next upstream transmission by applying a bandwidth allocation algorithm on the basis of the information. That is, in ATM-PON as described above, the upstream and downstream frames are constructed based on the ATM cells having a fixed size. However, in a network in which IP, such as GE-PON, is an important transmission protocol, a grant/request transmission or message transmission on the basis of a fixed-size cell is inefficient.
Accordingly, a new system based on a variable packet size is needed in the transmission efforts using the Ethernet frame suitable for IP.